Thermal ink-jet printing processes employ inks that are solid at room temperature and liquid at elevated temperatures. Such ink compositions include ink vehicles chosen to have a melting point above room temperature so that the ink compositions, which are melted in the apparatus, will not be subject to evaporation or spillage during periods of non-printing. The vehicles also possess low critical temperatures that permit the use of the solid ink in a thermal ink-jet printer. In thermal, or hot-melt, ink-jet printing processes employing these phase-change inks, the solid ink is melted by a heater in the printing apparatus and used as a liquid in a manner similar to that of conventional piezoelectric or thermal ink-jet printing. Upon contact with the printing substrate, the molten ink solidifies rapidly, enabling the dye to remain on the surface. Because the dye is not carried into the substrate by capillary action, higher print density than is generally obtained with liquid inks can be achieved.
Phase-change inks are desirable for ink-jet printers because they remain in a solid phase at room temperature during shipping, long-term storage, and the like. In addition, the problems associated with nozzle clogging as a result of ink evaporation with liquid ink-jet inks are largely eliminated, thereby improving the reliability of the ink-jet printing. Further, in phase-change ink-jet printers wherein the ink droplets are applied directly onto the final recording substrate (for example, paper, transparency material, and the like), the droplets solidify immediately upon contact with the substrate, so that migration of ink along the printing medium is prevented and dot quality is improved.
Hot-melt inks typically used with ink-jet printers have a wax-based ink vehicle, e.g., a crystalline wax. Such solid ink-jet inks provide vivid color images. In typical systems, these crystalline wax inks partially cool on an intermediate-transfer member and are then pressed into the image receiving medium such as paper. Transfuse spreads the image droplet, providing a richer color and lower pile height. The low flow of the solid ink also prevents show-through on the paper.
In these systems, the crystalline-wax inks are jetted onto a transfer member, for example, an aluminum drum, at temperatures of approximately 130 to 140° C. The wax-based inks are heated to such high temperatures to decrease their viscosity for efficient and proper jetting onto the transfer member. The transfer member is at approximately 60° C., so that the wax will cool sufficiently to solidify or crystallize. As the transfer member rolls over the recording medium, e.g., paper, the image comprised of wax-based ink is pressed into the paper. The images produced with inks composed of crystalline waxes are visually appealing; however, lowering the temperature at which the inks are jetted and improving the robustness of the printed images would be beneficial.
However, the brittle waxes used in inks such as those described above do not provide robust images and are easily scratched. Low viscosity, inks, such as those curable by ultraviolet (UV) radiation, provide a printing option that is both jettable and curable to robust image on paper. These inks lack the thermally driven change in viscosity of hot-melt inks required to successfully transfuse the image as well as prevent image show-through on paper. In addition, a UV-curable resin removes the requirement for a hard-wax ink vehicle. The resin can be cured to a tougher material than could ever be found with a wax. However, the transfuse drum makes use of the post-jetting solidification of the wax to preserve dot integrity during image build up and transfer.
The preponderance of functionalized materials useful for UV curing are difunctional. Multifunctionality insures that the desired cross-linked network will be achieved. In the dominantly used class, acrylates, three major classes exist: polyethers, polyesters, and polyurethanes. All contain oxygen and/or nitrogen in the backbone. Only the polyethers which are built up from ethylene and propylene glycols have the ability to be of sufficiently low viscosity to be the major component of jettable inks. There are very few long hydrocarbon-chain acrylate-monofunctional monomers and no commercial examples of a difunctional acrylate with long hydrocarbon chains.
While hot-melt ink compositions are used successfully, a need remains for phase-change ink compositions that are suitable for hot-melt ink-jet printing processes, such as piezoelectric ink-jet printing processes and the like. There is still a need for ink compositions that can be processed at lower temperatures and with lower energy consumption, and a need for inks that have improved robustness and printing latitude. There is also a need for ink compositions that have improved jetting reliability and latitude with respect to meeting both the jetting and transfuse requirements of curable aqueous and non-aqueous inks. In addition, a need remains for phase-change ink compositions that exhibit desirably low viscosity values at jetting temperatures. Additionally, a need remains for phase-change ink compositions that generate images with improved look and feel characteristics. Additionally, there is a need for phase-change ink compositions that generate images with improved hardness and toughness characteristics. A need also remains for phase-change ink compositions that are suitable for high speed printing, thereby enabling transaction and production printing applications. In addition, there remains a need for curable ink compositions for piezoelectric ink-jet printing that produce a stable image that can be transferred to a substrate without cracking and hardened upon cure.